1. Field of the Invention
The present invention relates to a sintered silicon carbide and a method for producing the same. More particularly, the present invention relates to a sintered silicon carbide and a method for producing the same which has a high density, exhibits electric conductivity, and is advantageously used as structural parts in parts of apparatuses for producing semiconductors, parts of electronic information processing instruments, and vacuum apparatuses this sintered silicon carbide. The present invention also relates to a sintered silicon carbide having high uniformity of electric conductivity and exhibiting stable electric conductivity.
2. Description of the Related Art
Silicon carbide is a strongly, covalently bonded compound and has heretofore been used in various fields by taking advantage of excellent properties such as excellent strength at high temperatures, heat resistance, wear resistance, and chemical resistance. Since the temperature of wafer treatments, the diameter of wafers, and the lot size in treatments have been increasing recently in the fields of materials for electronic information processing instruments and materials for producing semiconductor, there has been a need for a sintered silicon carbide which has excellent heat resistance, a high density, and a high purity, and which does not show deformation by heating or deterioration of properties by cleaning with chemicals such as hydrofluoric acid, like conventional products made of quartz.
It has been known that p-doping and n-doping of a silicon carbide can be achieved by substituting elements in the compound with an element of Group IIIb or Vb to form a substitutional solid solution, and that silicon carbide can be relatively easily provided with electric conductivity.
Since silicon carbide is a strongly covalently bonded compound as described above, it is not easy to sinter silicon carbide. The hot press process, the reaction sintering process, and the sintering process at an ordinary pressure have been known as the process for producing silicon carbide having a high density.
The hot press process comprises a step of sintering silicon carbide under elevated pressure. Since an auxiliary sintering agent containing aluminum was reported in an early paper as a metallic auxiliary sintering agent (J. Am. Ceram. Soc., 39(1956), No. 11, Pages 386-389, various types of metallic auxiliary sintering agent have been studied. Among them, a highly heat conductive and electric insulating sintered silicon carbide was developed in 1980 by the hot press sintering process with addition of BeO ("Silicon Carbide Ceramics", Pages 327 to 343, published by UCHIDA ROKAKUHO, 1988).
The reaction sintering process comprises (1) a step of mixing raw materials in which powder of silicon carbide and powder of carbon are mixed together, (2) a step of forming, (3) a step of reaction sintering, and optionally, (4) a step of a post-treatment. In this process, carbon particles in a shape formed in step (2) are silicified in step (3). This process has been used for producing parts of semiconductors because of advantages that change in dimension of the product is small, that no auxiliary sintering agent is necessary, and that a sintered product having a high purity can be easily obtained. However, a sintered product obtained in accordance with this process contains unreacted metallic silicon, and applications to parts and jigs used in the fields requiring heat resistance, chemical resistance, and high strength are limited.
The sintering process at an ordinary pressure was proposed by S. Prochazka in "Ceramics for High Performance Applications", Page 239, in 1974 in which a metallic auxiliary sintering agent is used to obtain silicon carbide. Structural materials having a high strength at high temperatures and a high density can be obtained in accordance with this process, and studies on silicon carbide have been developed. For the auxiliary sintering agent, there are two types of auxiliary sintering agent, i.e., a metallic auxiliary sintering agent such as a metal, for example, boron, aluminum and beryllium and a compound thereof, and a carbon auxiliary sintering agent such as carbon black and graphite. These are used in combination. It is a hypothesis that the function of the metallic auxiliary sintering agent, particularly the function of boron which is used as the most preferable auxiliary sintering agent, is to decrease the grain boundary energy by segregation at the grain boundary, to accelerate diffusion of carbon-boron substances at the grain boundary, and to suppress diffusion at the surface. It is also a hypothesis that the function of the carbon auxiliary sintering agent is to remove oxidized layers at the particle surface of silicon carbide. However, the hypotheses have not been fully considered.
Since the metallic auxiliary sintering agents used above cause elution of metal impurities during use at a high temperature or during cleaning with chemicals, these agents are not suitable when the sintered product is used in the field of apparatuses for producing semiconductors.
As a means to solve the above problems, a process was proposed in Japanese Patent Application Laid-Open (JP-A) No. 60-108370, in which a specific ultra-fine powder of silicon carbide obtained by decomposition of a silane compound is used, and a sintered product having a high density can be obtained by the hot press process without using any auxiliary sintering agent. However, properties of the obtained sintered product are not shown. In relation to this process, it is described in "Silicon Carbide Ceramics", Page 89, published by UCHIDA ROKAKUHO, 1988, that addition of boron (as an auxiliary sintering agent) is indispensable even when a powder prepared as described above is used.
As an improvement of the hot press process, a process was proposed in JP-A No. 2-199064, in which an ultra-fine powder of silicon carbide synthesized by the CVD hot plasma process is used, and a sintered product having a high density can be obtained by the hot press process without using any auxiliary sintering agent. However, the product prepared in accordance with this process cannot solve the above problems because the product contains several ppm or more of impurities such as iron and has insufficient quality, the ultra-fine powder of silicon carbide which has an average particle diameter of 30 nm and is considered to have the function of an auxiliary sintering agent is expensive, and an ultra-fine powder such as the powder used in this process requires a great deal of care in handling to prevent oxidation of the surface.
Therefore, it is difficult to obtain a sintered silicon carbide in accordance with a conventional process, which has a high density and a small content of impurities and is suitable for use of parts of apparatuses for producing semiconductors and parts of electronic information processing instruments. No sintered silicon carbide satisfying the above requirements has been commercially available.
Since silicon carbide is a well-known compound semiconductor and exhibits the electric insulation because of a very large band gap, it is necessary that conductive electrons should be doped into sintered silicon carbide in order to provide silicon carbide with stable electric conductivity. The electric conductivity is generally provided by being doped the sintered silicon carbide with impurities such as elements of Group III such as B, Al, and Ga (p-dopants) and elements of Group V such as N, P, and As (n-dopants). However, it is known that metallic elements adversely affect processes for producing semiconductors. Nitrogen is an only nonmetallic element among the above elements. Conventional processes for producing sintered silicon carbide requires a metallic auxiliary sintering agent such as boron as described above, and this cause a problem that the sintered silicon carbide is contaminated with the metal when the sintered silicon carbide is applied to parts of apparatuses for producing semiconductors and parts of electronic information processing instruments.
JP-A No. 7-53265 discloses a process for producing a silicon carbide ceramic material which is a N-type semiconductor containing nitrogen as a solid solution and contains, as the main component, silicon carbide containing 90% or less of the 6H crystal form in the entire crystal forms. Sintered silicon carbide prepared in accordance with this process exhibits very stable volume resistivity in the range from a room temperature through 500.degree. C. However, no description can be found with respect to density. It is also described that an auxiliary sintering agent such as boron or carbon is added, if necessary. In this process, a formed material is preliminarily sintered in a furnace under an argon atmosphere at 1950.degree. C. and then fully sintered under a nitrogen atmosphere at a higher temperature. Thus, similarly to a process proposed in Japanese Patent Publication (JP-B) No. 61-56187, this process is not regarded as a convenient process for producing a sintered silicon carbide having a high purity.
These process commonly using a solid solution containing nitrogen are conducted by sintering under a nitrogen atmosphere. As long as the sintering is conducted under an elevated pressure such as the hot press process, the amount of nitrogen contained in the solid solution may be different in outer portions and inner portions of the sintered material because of the gradient concentration of nitrogen. In other words, there is the possibility that the amount of nitrogen contained in the solid solution is different at various portions of the obtained sintered product. This phenomenon is considered to appear more markedly as the size of a sintered product increases. When such a sintered product is used as an electrical heating element, electric conductivity is different at various portions of the element because of the difference in the resistance, and the element cannot exhibit an excellent performance. When this sintered product is cut to a plurality of pieces, the produced pieces exhibits different electric conductivities, and products having the same quality cannot be obtained.